1. Field of the Invention
This invention relates to a magnetic carrier and a two-component developer which are used in an electrophotographic system, an electrostatic recording system or an electrostatic printing system.
2. Description of the Related Art
In recent years, in order to achieve high image quality and high running performance which are required in electrophotography and so forth, a resin-filled carrier is proposed in which the ferrite core material having pores is filled with a resin (see Japanese Patent Laid-open Applications No. 2007-57943 and No. 2006-337579). According to these proposals, the carrier can be made low in specific gravity and this can keep, to a certain extent, inferior images from being formed.
However, in a system making use of an a-Si drum in order to achieve the high running performance, the a-Si drum has a higher electrostatic capacity than any OPC drum, and hence a toner must be more triboelectrically charged than ever. Such a carrier, however, have had an insufficient triboelectric charge-providing ability, and hence, where images are printed after leaving for a week in a high-temperature and high-humidity environment (temperature 30° C./humidity 80% RH), the toner may stick to non-image areas to cause a phenomenon of the formation of inferior images (i.e., fog). Hence, it has been difficult in some cases to apply such a carrier to the system making use of an a-Si drum. Furthermore, where images are printed on 50,000 sheets at a low image density in an image percentage of 1%, any broken carrier particles may stick to images on a photosensitive drum (carrier sticking).
In addition, in order to achieve the high image quality it is necessary to keep a phenomenon (ring marks) from occurring in which ring-like or spot-like patterns appear on recording sheets. The ring marks concern a phenomenon which comes about because any low-resistance foreign matter is present on a developer carrying member to cause the leaking of electric charges from the developer carrying member to the photosensitive drum. To prevent it, the peak-to-peak voltage (Vpp) of an alternating bias must be set low. However, it has turned out that, if the Vpp is set low in using the carrier disclosed in Japanese Patent Laid-open Applications No. 2007-57943 and No. 2006-337579, a low developing performance may result to cause a decrease in image density. It has further come about that the toner at the rear end of a halftone area is scraped off at the boundary between the halftone area and a solid area to make white lines, to cause image defects (blank areas) in which edges of solid areas stand emphasized.
Meanwhile, a carrier is proposed which is obtained by forming, in a supercritical fluid, coat layers on ferrite cores in the state a resin is dissolved or dispersed, so as to make the resin small in standard deviation of its layer thickness (see Japanese Patent Laid-open Application No. 2007-72444). The use of this carrier enables formation of high-density images in an image forming apparatus having a process speed of about 200 mm/sec. However, in a high-speed machine having, e.g., a process speed of 300 mm/sec or more, which is adaptable to POD (print on-demand), there has been a problem that the blank areas occur because of an insufficient developing efficiency. In such a high-speed machine having a process speed of 300 mm/sec or more, it has also come about in some cases that, where images are printed on 50,000 sheets in an image percentage of 1%, the resin layers at the surfaces of such magnetic carrier particles deteriorate to cause variations in image density before and after running.
Also proposed are a carrier the ferrite cores of which have been so coated with a resin as to have surface unevenness coming from fine crystal particles, and a carrier the ferrite cores of which have been incorporated with a resin only at their concavities (see Japanese Patent Laid-open Applications No. H04-93954 and No. S58-216260). According to these Japanese Patent Laid-open Applications No. H04-93954 and No. S58-216260, carriers can be obtained which have been improved in environmental dependency and toner-spent resistance to a certain extent. However, the layer thickness of the resin is not controlled, and hence there has been a problem that, in a normal-temperature and low-humidity environment (temperature 23° C./humidity 5% RH), the blank areas occur because of a lowering of developing efficiency when the Vpp is set low.